Do you like math puzzles? Here:

[drray23]

3. I don't think its math.

it is more of an issue of what the purpose is. If your cardiologist asked you to reach a specific intensity or burn a set number of calories.

You can look at this https://www.healthline.com/health/calories-burned-walking

to give you an idea. at 2miles per hour, look up the first column and you will get an approximate number for your weight.

Then use that
https://www.bostonrockgym.com/how-many-calories-burned-on-recumbent-bike/

to see the equivalent for a recumbent bike to reach the same intensity.

If he asked instead for a specific heart rate just measure it when exercising and match that on your recumbent bike.